The present invention is a method of making aromatic chlorosilane compounds, having important industrial application as silane coupling agents and for modifying silicones, by hydrosilating an aromatic vinyl compound with a hydridochlorosilane compound in the presence of a platinum or platinum compound catalyst and a carboxylic acid compound, which imparts a high positional selectivity to the hydrosilation reaction.
Functional chlorosilanes are silicon-containing compounds which find important industrial applications as raw materials for modifying silicones and as silane coupling agents. In order to impart to silane coupling agents and modified silicones an improved resistance to heat, to control their refraction indices, and to improve their compatibility with organic compounds and polymers, aromatic substituents are often introduced into their structures. The following are examples of compounds that contain aromatic groups: phenyl silicon compounds in which silicon is bonded directly to an aromatic ring, e.g., phenylsilane compounds and phenylsilicone derivatives; compounds in which aromatic groups are bonded to silicon via hydrocarbon groups, e.g., a benzylsilane compound; and silicone derivatives having 3-phenylpropyl groups or phenethyl groups, or similar aralkyl-type silicon compounds.
Among the above, phenyl groups which are bonded directly to silicon are formed as a result of a reaction between silicon metal and chlorobenzene (so-called "direct process"), a reaction of removal of hydrogen from benzene and hydridochlorosilane in the presence of a boron chloride catalyst, or an equivalent organic reaction such as a Grignard method. Aralkyl silicon bonds can also be produced by an equivalent organic reaction such as a Grignard method. However in the case of 3-phenylpropyl groups or phenethyl groups, it would be more economical to utilize a reaction of hydrosilation of aromatic compounds such as styrene or a similar compound having unsaturated groups. In particular, many vinyl substituents of styrene, naphthalene, pyridine, or similar aromatic compounds, as well as many vinyl substituents of derivatives of styrene, naphthalene, and pyridine are known, commercially available, and suitable for synthesis of silicon compounds having aromatic substituents.
It is known that hydrosilation of aromatic compounds having a vinyl substitution is normally accompanied by the formation of both .alpha.- and .beta.-adducts to the aromatic ring. In particular, when a hydridochlorosilane is used as the silane the .beta.-adducts are more readily hydrolyzed than the .alpha.-adducts and therefore the formation of .beta.-adducts is typically more desirable. Another reason it is preferable to use methods for synthesis of .beta.-adducts alone is that it is not easy to separate .alpha.- and .beta.-adducts from a mixture.
It has been reported that the above problem can be solved by modifying a platinum catalyst with a ligand of a phosphorus compound such as an organic phosphine. However, since the use of such a phosphorus-modified platinum catalyst noticeably reduces activity, the reaction requires a long time and high temperature. Therefore, this method of solving the above mentioned problem is not entirely satisfactory since vinyl groups which are bonded directly to aromatic groups normally have high polymerization activity, and, if the hydrosilation reaction is carried out for a sufficiently long time and at high temperature polymerization of vinyl groups occurs whereby the vinyl compound is lost.
It is an object of the present invention to increase the amount of .beta.-adducts to aromatic rings in a method of making an aromatic chlorosilane compound by hydrosilation of an aromatic compound having a vinyl group directly bonded to the aromatic ring (hereinafter referred to as "aromatic vinyl compound") with a hydridochlorosilane compound. It is another object to provide a method of making an aromatic trichlorosilane compound comprising the .beta.-adduct essentially without making the .alpha.-adduct.